1. Field of the Invention
The present invention relates to a drive circuit of a switch and a relay circuit.
2. Background Art
Conventionally, mechanical relays and photo relays have been used as switching devices. Recently, devices having mechanical contacts driven by static electricity, in particular MEMS (Micro-Electro-Mechanical System) relays which have switch contacts using MEMS, are attracting attention as such switching devices, as disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2002-236265. Since MEMS switches used in such MEMS relays are small in size and light in weight, they can operate at a high switching frequency. In particular, MEMS switches driven by static electricity are faster in displacement rate and lower in power consumption than MEMS switches driven by heat. For this reason, MEMS relays using MEMS switches driven by static electricity are attracting attention as key devices of mobile terminals, for which low power consumption is required, wireless antennas, for which low insertion loss is required, high-speed wireless communication, for which high-frequency characteristics are required, etc.
In MEMS switches that are driven by static electricity, a high voltage is necessary to generate an electrostatic force sufficient for driving a contact. For this reason, in order to obtain a reliable switching performance, the electrostatic driving requires a high voltage, which makes it difficult to decrease the size and the cost of the drive circuit (relay circuit).
In order to solve this problem, the present inventors have studied a method of using a photoelectromotive force produced by a photodiode array to generate a voltage necessary to drive an MEMS switch. In this method of using a photoelectromotive force produced by a photodiode array, small photodiodes are connected in series, and a high voltage can be obtained by increasing the number of the photodiodes connected in series. A drive circuit which is smaller in size and lower in cost can be obtained in this manner.
As described above, devices having mechanical contacts driven by static electricity, in particular MEMS relays including switch contacts using MEMS, are attracting attention as switching devices which are superior in high-frequency characteristics, etc. However, unlike contacts using semiconductor devices such as MOSFETs, etc., mechanical switch contacts are mechanical reed type contacts. Accordingly, there is a problem in that the long-term reliability and lifetime of a MEMS relay are inferior to those of a semiconductor device. In particular, one of the most critical problems that shorten the lifetime of an MEMS switch is a movable part sticking to a substrate. The cause of this problem has not been clarified sufficiently.
Such a problem of limited lifetime also occurs in the aforementioned switching device that the present inventors have studied, which is manufactured by combining an MEMS switch driven by static electricity and a relay circuit using a photodiode array. In order to solve this problem of limited lifetime, the present inventors have conducted various studies and analyses.